A viewing angle adjustable display may have two viewing modes, a public mode and a private mode. In the public mode the device commonly behaves as a standard display. A single image is displayed by the device to as wide a viewing angle range as possible, with optimum brightness, image contrast and resolution for all viewers. In the private (or privacy) mode, the main image is discernable only from within a reduced range of viewing angles, usually centred normally to the display. Viewers observing the display from outside the reduced angular range will perceive either a masking, side image which obscures the main image, or a main image so degraded as to render it unintelligible.
Several types of viewing angle adjustable displays, with varying degrees of additional cost over a standard display, ease of use and strength of privacy performance are well known.
Devices incorporating such viewing angle adjustable displays include mobile phones, eBook readers, Personal Digital Assistants (referred to herein as PDAs), laptop computers, desktop monitors, Automatic Teller Machines (referred to herein as ATMs) and Electronic Point of Sale (referred to herein as EPOS) equipment. Viewing angle adjustable displays can also be beneficial in situations where it is distracting, and therefore unsafe, for certain viewers to observe certain images at certain times. For example an in car television screen should not be observed by a driver whilst the car is in motion.
Several methods exist for adding a light controlling apparatus to a display with a naturally wide viewing angle. One such light controlling apparatus is the microlouvre film described in U.S. RE27617 (Olsen; Aug. 15, 1967), U.S. Pat. No. 4,766,023 (Lu; Aug. 23, 1988) and U.S. Pat. No. 4,764,410 (Grzywinski; Aug. 16, 1988). This and other methods involving detachable optical arrangements are not conveniently switchable as changing the display between the private and public modes requires manual placement and removal of the film or other apparatus.
Methods of providing an electronically switchable viewing angle adjustable display are disclosed in GB2413394 (Winlow et al.; Oct. 26, 2005), WO06132384A1 (Kean et al.; Dec. 14, 2006) and GB2439961 (Smith et al.; Jan. 16, 2008). These inventions describe switchable privacy devices constructed by adding one or more extra liquid crystal layers and polarisers to a display panel. The intrinsic viewing angle dependence of these extra elements can be changed by switching the liquid crystal electrically in a known way. Devices utilising this technology include the commercially-available Sharp Sh851i and Sh902i mobile phones. These methods share the disadvantages that the additional optical components add thickness and cost to the display.
Methods to control the viewing angle properties of a liquid crystal display (referred to herein as LCD) by switching the single liquid crystal layer of the display between two different configurations, both of which are capable of displaying a high quality image to the on-axis viewer are described in US20070040780A1 (Gass et al., Feb. 22, 2007) and GB2455061 (Broughton et al.; Jun. 3, 2009). These devices have the advantage that they provide the switchable privacy function without the need for added display thickness, but have the disadvantage that they require complex pixel electrode designs and other manufacturing modifications compared to a standard display.
One example of a display device with privacy mode capability with no added display hardware complexity is the commercially-available Sharp Sh702iS mobile phone. This uses image data manipulation in conjunction with the angular data-luminance properties inherent to the liquid crystal mode used in the display, to produce a private mode in which the displayed information is unintelligible to viewers observing the display from an off-axis position. A key advantage of this type of method is that in the public mode, the display consists of, and operates as, a standard display, with no image quality degradation causes by the private mode capability. However, when in the private mode, the quality of the image displayed to the legitimate, on-axis viewer is reduced.
GB2428152A1 (Wynne-Powell et al.; Jan. 17, 2007), WO2011034209 (Broughton et al.; Mar. 24, 2011) and WO2011034208 (Broughton et al.; Mar. 24, 2011) disclose improved schemes where the image data is manipulated in a manner dependent on a second, side image. Consequently, in the private mode, an on axis viewer observes the main image whilst an off-axis viewer observes the side image. These methods provide an electronically switchable public/private display with no additional optical elements required, minimal cost and satisfactory privacy performance. However these methods can suffer from a distracting on-axis flash artefact when the side image changes significantly and suddenly from one frame to the next.
WO2009110128A1 (Broughton et al.; Sep. 11, 2009) discloses a method that solves the on-axis flash artefact problem by graduating the transition of the side image from dark to bright or vice versa. Inserting two image frames with intermediate side image luminance values between the bright and dark side image states, minimises the effect of the flash to the on-axis observer. However inserting two intermediate frames reduces the possible video frame rate of the side image which is undesirable.
It is therefore desirable to provide a high quality LCD display which has public and private mode capability, in which no modifications to the LC layer or pixel electrode geometry is required from a standard display, has a substantially unaltered display performance (brightness, contrast ratio, resolution, etc.) in the public mode and, in the private mode has a strong privacy effect with minimal degradation to the on-axis image quality, particularly for a changing side image.